Commercially available compositions serving a variety of industrial needs for creating image patterns have become recognized as a distinct class of materials, referred to as photoresists. Photoresists are coated or otherwise spatially positioned, imagewise exposed to actinic radiation, and developed to leave the photoresist selectively in either exposed or unexposed areas. The name "photoresist" originated from the use of material remaining after imaging and development to resist etching and thereby define an etching pattern. However, today photoresists are employed for many divergent applications, including the formation of totally exposed layers, such as planarizing layers, and producing colored images.
Negative working photoresists are those which upon development are retained in areas exposed to actinic radiation. Negative working photoresists characteristically include an organic film forming component exhibiting ethylenic (vinyl) unsaturation. The film forming component is rendered immobile on development by undergoing photoinduced addition reactions at the ethylenic unsaturation sites.
The film forming components containing ethylenic unsaturation generally exhibit limited direct response to imaging radiation and therefore rely on one or more initiators for the ethylenic addition reaction. Heseltine et al and Jenkins et al U.S. Pat. Nos. Re. 27,922 and Re. 27,925 disclose N-oxyazinium initiators for photocrosslinking and photopolymerization, respectively.
It is common practice in preparing photoresist compositions to employ coinitiators. One of the coinitiators is a photosensitizer. Photosensitizers are relied upon to capture photons of exposing radiation. The remaining coinitiator is referred to as an activator. The activator is not relied upon to respond directly to exposing radiation, but rather adjacent activator and photosensitizer molecules react, following excitation of the latter by photon capture, causing release of a free radical. Released free radicals induce immobilizing addition reactions at sites of ethylenic unsaturation.
Photoresists are most commonly imagewise exposed at ultraviolet wavelengths. The necessity of using shorter imaging wavelengths places constraints on the master or pattern employing for imaging. For example, a master which presents a visible dye image, but exhibits little variance in ultraviolet transmittance is not well suited for imagewise exposing an ultraviolet responsive photoresist.
There are several difficulties in attempting to formulate photoresists which respond to wavelengths longer than 550 nm. For example, the energy contained in light photons declines as the wavelength of the radiation increases. This places increased demands on any photosensitizer coinitiator intended to adsorb in longer wavelength regions of the spectrum. Additionally, generally more elaborate molecules are required to absorb longer wavelength radiation. This increases the bulk of the photosensitizer. Related to this, reduced thermal and storage stability have been a point of concern.
It is generally accepted that photosensitizer coinitiators function by photon absorption to lift an electron from an occupied molecular orbital to a higher energy, unoccupied orbital. The spin of the electron lifted to the higher energy orbital corresponds to that which it exhibited in its original orbital or ground state. Thus, the photosensitizer in its initially formed excited state is in a singlet excited state. The duration of the singlet excited state is limited, typically less than a few nanoseconds. The excited photosensitizer can return from its singlet excited state directly to its original ground state, dissipating the captured photon energy. Alternatively, the singlet excited state photosensitizer in some instances undergoes intersystem crossing, through spin inversion to another excited state, referred to as a triplet state, wherein lifetimes are typically in the microsecond to millisecond range. Since photosensitizer coinitiators which exhibit triplet states remain in an excited state for time periods that are orders of magnitude greater than photosensitizer coinitiators which exhibit only singlet excited states, a much longer time period is available for reaction with the paired activator coinitiator.
Specht and Farid U.K. No. 2,083,832A discloses photopolymerization coinitiators including azinium activators and amino-substituted photosensitizer (e.g., amino-substituted ketocoumarin) coinitiators which exhibit triplet states on excitation. An essentially cumulative disclosure is provided by Research Disclosure, Vol. 200, Dec. 1980, Item 20036. Research Disclosure is published by Kenneth Mason Publications Ltd., Emsworth, Hampshire P010 7DD, England. As illustrated by Specht, Martic, and Farid, "A New Class of Triplet Sensitizers", Tetrahedron, Vol. 38, pp. 1203-1211, 1982, these amino-substituted 3-ketocoumarins exhibit intersystem crossing efficiencies ranging well above 10 percent--e.g., from 0.18 to 0.92 or 18 to 92 percent, measured in polymer. These photosensitizers were chosen for their ability to form triplet states on excitation and generally absorb exposing radiation of shorter wavelengths, the longest wavelength absorption maxima reported being 480 nm.
In concurrently filed, commonly assigned patent application, Ser. No. 933,658, titled NEGATIVE WORKING PHOTORESISTS RESPONSIVE TO SHORTER VISIBLE WAVELENGTHS AND NOVEL COATED ARTICLES, negative working photoresists are disclosed comprised of a film forming component containing ethylenic unsaturation and capable of selective immobilization as a function of ethylenic addition, and activator and photosensitizer coinitiators for ethylenic addition. The activator is an azinium salt, and the photosensitizer is a yellow dye having a reduction potential which in relation to that of the azinium salt activator is at most 0.1 volt more positive, with the further proviso that, when the photosensitizer is a yellow keto dye, it exhibits when excited by imaging radiation an intersystem crossing efficiency to a triplet state of less than 10 percent.
In concurrently filed, commonly assigned patent application, Ser. No. 933,712, titled DYE SENSITIZED PHOTOGRAPHIC IMAGING SYSTEM, a photographic imaging system comprised of an imaging dye or a precursor thereof, a hardenable organic component containing ethylenic unsaturation sites and capable of imagewise modulating mobility of the dye or dye precursor as a function of addition at the sites of ethylenic unsaturation, and coinitiators for ethylenic addition. The coinitiators are comprised of of an azinium salt activator and a photosensitizer which is a dye exhibiting a reduction potential which in relation to that of said azinium salt activator is at most 0.1 volt more positive, with the further proviso that, when the photosensitizer is a yellow keto dye, it exhibits when excited by imaging radiation an intersystem crossing efficiency to a triplet state of less than 10 percent.
In concurrently filed, commonly assigned patent application, Ser. No. 933,657, titled ENHANCED IMAGING COMPOSITION CONTAINING AN AZINIUM ACTIVATOR, an imaging composition is disclosed comprised of an organic component containing ethylenic unsaturation sites and capable of selective hardening by addition at the sites of ethylenic unsaturation, and azinium salt activator, a photosensitizer having a reduction potential which in relation to the reduction potential of the azinium salt activator is at most 0.1 volt more positive, and an image enhancing amount of benzene substituted with an electron donating amino group and one or more groups capable of imparting a net Hammett sigma value electron withdrawing characteristic of at least +0.20 volt to said benzene ring.